The invention relates to a two-stroke engine with pneumatic fuel injection and restriction of a flowrate at the exhaust.
Two-stroke engines with one or more cylinders generally have, associated with each of the cylinders, a crankcase called a pump-crankcase communicating with one end of the cylinder and allowing introduction of fresh gas into the cylinder through at least one pipe and one transfer port. The reciprocating piston in the cylinder also provides intake and compression of fresh gases in the pump-crankcase.
An intake valve located on the pump-crankcase allows fresh gases to enter the crankcase when the piston moves in the direction away from the crankcase. The fresh gases are then compressed and close the valve when the piston moves toward the crankcase. When the corresponding cylinder ports are cleared by the piston, fresh gases are introduced into the cylinder via the pipes and transfer ports and scavenge the burned gases which are discharged through exhaust ports generally disposed slightly offset with respect to the transfer ports.
The piston moves away from the crankcase to compress the gases contained in the cylinder. Ignition and combustion of the mixture then cause the power stroke of the piston toward the crankcase.
French Patent FR-2.496.757 proposed providing a device for pneumatic injection of fuel into the cylinder using the pressure of the fresh gases inside the pump-crankcase. To accomplish this, a means of dispensing liquid fuel is connected directly to the pipe from the pump-crankcase. The compressed air in the pump-crankcase fed to the dispenser through the pipe ensures atomization and injection of the fuel into the cylinder.
A number of improvements have been made to the aforementioned proposed device and it has been proposed, for example, to provide a volume in the pipe connecting the pump-crankcase to the injector and a valve at the end of the pipe connected to the pump-crankcase. This provides a compressed-air reserve at a pressure close to the maximum pressure in the pump-crankcase during the cycle. This compressed-air reserve then serves to atomize the fuel and introduce it into the cylinder in the form of a gas and air mixture when the injector is actuated.
One of the drawbacks of two-stroke engines is the loss of efficiency due to the fact that the fresh gases mixed with air are insufficiently separated from the gases burned inside the cylinder and are hence responsible for creating conditions unfavorable to initiation of combustion.
To remedy this drawback, it has been proposed that a flowrate restriction element be placed in the engine exhaust pipe in the vicinity of the cylinder to slow down the discharge of burned gases. This provides stratification of the fresh gases and the burned gases since the fresh gases are driven back into the zone of the cylinder where injection and ignition take place. However, such an exhaust flowrate restriction technique has never been used in the case of a two-stroke engine having a pneumatic injection device using the compressed air from the pump-crankcase.
On the other hand, in the case of pneumatic injection using compressed air from the pump-crankcase, the pressure differential between the air used for injection and the gases filling the cylinder at the time of injection is generally too small to ensure good atomization of the fuel and high injection efficiency. This drawback remains significant when a volume associated with the injector and a pipe separated by a pump-crankcase valve are used. The pressure in the volume, which is at most equal to the maximum pressure in the pump-crankcase, is not sufficiently higher than the pressure in the cylinder at the time of injection, so that the pressure in the cylinder is increased by wave effects in the exhaust pipe.
Hence, the goal of the invention is to propose, according to a first embodiment, a two-stroke engine with at least one cylinder in which the piston moves, with a crankcase communicating with one end of the cylinder and having a means for admitting air into the crankcase. At least one pipe is provided for admitting air not mixed with fuel into the cylinder, which joins the crankcase to a cylinder transfer port with a device pneumatically injecting fuel into the cylinder comprising a pneumatic injector. A means on this pneumatic injector supplies and meters pressurized air for atomizing the fuel, and the pipe is connected to the crankcase through a valve and comprises a volume of pressurized air connected to the pneumatic injector. An exhaust pipe is connected to the cylinder via an exhaust port located offset in the direction of piston travel with respect to the transfer port of the cylinder, with this two-stroke engine having increased efficiency and improved function because of improved atomization and injection of fuel into the cylinder.
With this goal in view, the engine according to the invention also has, inside the exhaust pipe, in the vicinity of the cylinder, an element for restricting the passage of the exhaust gases, whose opening and closing are controlled and adjusted as a function of at least one engine operating parameter.
The present invention also relates to another embodiment according to which the two-stroke engine has a first cylinder in which a first piston moves, a second cylinder in which another piston moves, with a crankcase communicating with one end of said second cylinder. This engine also has a means for admitting air into the crankcase, at least one pipe for admitting non-mixed air into the first cylinder, with the pipe being connected to an opening in the first cylinder. A device is provided for pneumatic injection of fuel into the first cylinder comprising a pneumatic injector, with a means supplying and metering fuel to this pneumatic injector, and a pipe supplying the injector with pressurized air for atomization of the fuel. The pipe is connected to the crankcase, and an exhaust pipe is connected to the first cylinder by an exhaust port located in the wall of the first cylinder.
In addition, this engine has, inside the exhaust pipe, in the vicinity of the first cylinder, an element for restricting the passage of the exhaust gases coming from this cylinder, whose opening and closing are controlled and regulated as a function of at least one engine operating parameter.
There is a non-zero angular displacement between the cycles of these two cylinders. This angular displacement may be 120.degree., with the cycle of the first cylinder leading the cycle of the second cylinder by 120.degree.. The engine may have a number of cylinders which is a multiple of 3.
This angular displacement may be 90.degree., with the cycle of the first cylinder leading the cycle of the second cylinder by 90.degree.. The engine may have a number of cylinders which is a multiple of 4.
The pipe supplying the injector with pressurized air may connect said pneumatic injector directly to the crankcase. This means that this pipe will not include a volume for storage of pressurized gas.
As a result, the part of this pipe connecting the crankcase to the pneumatic injector may advantageously have no check valve and may be small in volume.